The process according to the invention relates to the removal of ammonium sulphate from tar-rich waste streams from the production of (meth)acrylonitrile by adding methanol and ammonia and separating off the ammonium sulphate thus precipitated out. By this means the residual waste stream is substantially freed from its sulphur content.
In an effort to minimize the emission of environmentally unfriendly gases, to which group SO.sub.2 also belongs, laws and regulations have been introduced which lay down emission limits. Thus, for example, for the Federal Republic of Germany it is specified in the "Technische Anleiting Luft" (TA Luft) ("Air Pollution Control Regulations") that the SO.sub.2 content of the flue gas from incinerator plants may not exceed a value of 100 mg/Nm.sup.3. The corresponding limit for oxides of nitrogen (NO.sub.x) is 500 mg/Nm.sup.3.
Through the use of sulphur-containing (waste-) fuels, flue gas SO.sub.2 concentrations of more than 1000 mg/Nm.sup.3 are frequently obtained. For this reason, in order to remove the SO.sub.2, for example in coal-fired power stations, a flue gas desulphurization is installed as an additional process step, in which, after an extensive scrubbing of the gas, the SO.sub.2 is converted to calcium sulphate (CaSO.sub.4) using lime. Because of the growing amounts of calcium sulphate being produced, it is becoming increasingly difficult to usefully dispose of it, for example as a component of building materials. Both investment and operating costs (use of lime amongst others) and disposal of calcium sulphate represent a high burden on energy costs (public energy supply) or production costs (industrial power stations).
In order to minimize this expenditure on an environmentally correct disposal, or even to render it unnecessary, the aim should be pursued of either preventing the production of the harmful substances or, if their formation cannot be avoided, carring out their environmentally correct removal at the earliest possible time, i.e. immediately after their production. The process according to the invention achieves such a solution to the problem of the disposal of ammonium sulphate-containing waste streams from the production of (meth)acrylonitrile.
In the production of acrylonitrile or methacrylonitrile by ammoxidation of propylene or i-butene excess ammonia must be removed from the reaction gas. The ammonia is converted to ammonium sulphate by addition of sulphuric acid; this ammonium sulphate either remains in the sulphate liquor or precipitates out as a crystalline slurry to be separated off.
This sulphate liquor also contains the oligomers and polymers which are unavoidable in the production of (meth)acrylonitrile.
Treatment of the mother liquor from the ammonium sulphate crystallization with methanol has already been tried (EP 126 058); the objective here was seen as being the recovery of additional qualitatively satisfactory ammonium sulphate. For this purpose the tar content (oligomers and polymers) should be at most 25%, preferably at most 20%. Consequently a larger quantity in absolute terms of water and ammonium sulphate is present. As a result the amount of methanol necessary for the fundamentally known salt precipitation also increases sharply. Nevertheless, it is not possible to suppress the residual salt content, and thus the residual sulphur content, to such an extent that on combustion of the residual liquor remaining after this operation the SO.sub.2 content of the combustion gases satisfies the TA Luft requirements. In the process of this EP 126 058 the ammonium sulphate obtained in this manner is dissolved in crude sulphate liquor and fed again into the main crystallization process, whilst the total residual liquor originating from the methanol treatment (after recovery of the methanol) is fed to an incinerator and increases the load on this.
It has now been found that it is more advantageous to separate the mother liquor from the main crystallization, in the form in which it is obtained after the separation of the ammonium sulphate, for example, as centrifugate, into a tar-rich waste stream and an essentially tar-free mother liquor by allowing it to settle.
This tar-rich waste stream can be separated off from the surface of the sulphate liquor, for example by skimming off, and treated in the process according to the invention. Such waste streams contain between 25 and 80% by of weight of oligomers and/or polymers from the production of (meth)acrylonitrile, typically 30-60% by weight. Since these tar-containing waste streams have stood in contact with the ammonium sulphate liquor, they still contain ammonium sulphate in an amount of 10-30% by weight, typically 15-25% by weight; if such waste streams are disposed of in an incinerator without pretreatment, these waste streams therefore contribute to an increase in the SO.sub.2 and NO.sub.x emissions. The remainder, to 100%, of these waste streams is essentially water. Waste streams of the said type are obtained as waste product in an amount of 20-80 kg/t of (meth)acrylonitrile, typically 25-40 kg/t.
The mother liquor, which for example, has been liberated by skimming off, is fed back into the main crystallization process.
The process according to the invention now makes it possible to remove virtually the entire sulphur content and additionally the major part of the nitrogen content from such waste streams and thus makes these waste streams easier to handle during their disposal.